1. Technical Field of the Invention
The present invention relates to circuit protection devices, and more particularly to electronic circuitry which functions in the manner of a conventional dual element fuse or thermal-magnetic circuit breaker to protect semiconductor power switches and wiring in electrical system for automobiles or other applications. The devices can quickly shut down the drive circuit of a power switch in response to a high fault current, and also to provide a timed shutdown according to an I2t function or other desired function in response to a lower level overload. The invention is described in the context of an automobile electrical system but those skilled in the art will recognize other semiconductor device applications with which the present invention can advantageously be employed.
2. Relevant Art
Fuses are well known wiring protection devices which provide substantially instantaneous tripping in response to high fault currents, e.g., above about 1000% of rating, or delayed tripping in response to overcurrents up to about 500% to 1000% of rating. So-called dual element fuses and mechanical circuit breakers are also well known devices which can provide both of these functions in a single device. This is advantageous where fast shutdown under short circuit conditions and the timed tripping to accommodate transient inrush currents are both needed. Fuses and circuit breakers can provide these functions acceptably in many instances, but there are applications for which conventional devices are not well suited.
For example, separate fast acting and time delay fuses are customarily used for protection of automotive electrical systems, although dual element fuses, when available for automotive applications.
Headlight circuits and motors are examples of applications requiring a time delay tripping capability. Automotive headlights are exposed to a wide range of temperatures, and when turned on at very low temperatures, can experience quite high transient inrush currents, e.g., ten times the normal operating current at a temperature of −40 C. Motors also generate high inrush (starting) currents.
Even apart from the fact that a supply of different types of fuses must be kept at hand in anticipation of a short circuit or overcurrent condition, replacement of blown fuses in an automobile is generally inconvenient because the fuses are often located out of the way in both the passenger and engine compartments.
Conventional circuit breakers do provide dual protective functions, and can easily be reset, but even the smallest thermal-magnetic circuit breakers, and the panels in which they are designed to be installed, are not small enough for practical use in automotive applications, and would have to be located inconveniently. Of even greater importance would be the cost of circuit breakers.
There exist components with a similar goal, that is provide a protection at a current lower than the inrush current. The philosophy is to set the protection current at a high level for a given time after the power switch is turned on, and to set the protection current at a lower level after this given time. The main drawback of this approach is that the behavior is only related to the time, and not to the energy dissipated. For example, if a light bulb with the switch on, the current protection will be low and the bulb will never turn on because the device will enter protection mode on the inrush current of the light bulb.
Also, for better overcurrent protection, control should be related the energy effectively dissipated.
A need therefore exists for a circuit protective device suitable for protection of switched semiconductor power devices in automotive applications and the like which can provide both instantaneous short circuit protection and time delayed tripping to accommodate transient inrush currents and does not have the disadvantages of conventional devices.